“Unseating” of a tire means at least one of the beads of the tire escaping from all or part of the seat of the rim on which the tire is mounted. The capacity of a tire to resist unseating is one of the features which is important to the safety of the users of a vehicle with tires, since unseating is associated with considerable deterioration of rolling performance. A study of the phenomenon has shown that there are two types of unseating: (1) unseating resulting from violent impacts to the sidewall or the tread of the tire, such as for example an impact against a pavement or passage of the tire over a hole in the roadway, and (2) unseating in the presence of very strong mechanical stresses such as for example during travel at high speed round a bend with a small radius of curvature, all this possibly being made worse by an inflation pressure lower than the operating pressure. The invention relates to a test method for quantifying the behavior of tires in relation to unseating of the second type.
A distinction may be drawn between two categories of test methods which make it possible to study and quantify unseating of a tire: methods for testing on machines and methods for testing on tracks. An example of a method for testing on machines is described in the patent U.S. Pat. No. 3,662,597, according to which a conical member is applied against a sidewall of the tire, then the pressure exerted against the sidewall is increased and displacement of the sidewall is recorded. A similar test method is to be found in Federal Motor Vehicle Safety Standard No. 109, drawn up by the NHTSA (National Highway Traffic Safety Administration), the American Federal road safety body.
Another device was proposed in patent application WO 02 090919: once again, a force is applied to the tire mounted on a rim and inflated to a given pressure, but unlike in the method described in document U.S. Pat. No. 3,662,597, the force is not applied to the sidewall of the tire, but to the part of the tire intended to be in contact with the ground, until unseating takes place.
Track tests are complementary to the tests on machines, since they place the tire under conditions which are closer to the conditions of use of a vehicle. Qualification of a tire by a vehicle manufacturer frequently depends, inter alia, on the results obtained in unseating tests on track. These tests are particularly critical when the tires in question are designed specifically to allow rolling at low or even zero pressure, hereinafter designated “run-flat tires”, without significant deterioration of rolling performance.
The unseating tests known from the prior art generally adopt the configuration known as “J turn” or “U turn”. As shown in FIG. 3, a vehicle fitted with the tire to be tested starts off at a constant speed along a straight line, subsequently entering a bend whose radius of curvature is predefined. Various runs are performed, with the tire pressure being reduced with each run until unseating occurs. U.S. Pat. No. 4,700,765 describes this type of test, performed at a speed of 50 km/h and with a radius of curvature of 20 meters, but other combinations have been proposed: 60 km/h and 25 meters (EP 334 955), 60 km/h and 30 meters (EP 377 338), 40 km/h and 25 meters (EP 1 036 675) and even 35.5 km/h and 6 meters (EP 376 137). A variant of the test comprises a full turn with a predefined radius of curvature (U.S. Pat. No. 5,253,692).
Under conventional test conditions, passenger-car tires not specifically designed to allow rolling at low or indeed zero pressure suffer unseating at inflation pressures of the order of 1 bar. When the tires in question are designed to allow rolling at inflation pressures of much less than 1 bar without significant deterioration of rolling performance, the method has to be adapted. The test is performed at an inflation pressure of zero; “J turn” runs are performed until unseating takes place, either at a constant speed (EP 377 338) or at progressively increasing speed (EP 334 955). The number of runs without unseating serves as an indicator of tire performance.
One of the difficulties of this approach is that the results cannot be reproduced satisfactorily, due to the fact that the steering angle during the bend is not controlled. “Steering angle” means the angle of rotation imposed by a driver on the steering wheel of a vehicle in order to modify the trajectory of the vehicle. In principle, the test driver has to keep the tire to be evaluated on the ideal trajectory (recommended radius of curvature), but the trajectory over which it actually travels is only ever an approximation thereof, as shown in FIG. 4. To be valid, the trajectory has to be situated within the limits formed by the circles of radius R+ and R−. A vast number of trajectories are possible within these limits: a driver may approach the bend by imposing an elevated steering angle and then correct this angle towards lower values, or conversely start the maneuver with a small steering angle, and then correcting it to remain within the limits. This maneuvering latitude results in weak reproducibility. Another drawback of the previous approach lies in its very low discriminatory capacity. Very often, it is used as a binary test (the tire either does or does not fulfill the criterion set) and does not allow the classification of tires which have fulfilled the criterion: thus it is impossible to distinguish between two types of tires which have resisted unseating after a recommended number of runs.
U.S. Pat. No. 6,580,980 provides greater reproducibility, by proposing a variant of the method which allows the problem of steering angle variation to be avoided. The driver of the vehicle traveling in a straight line suddenly changes the steering angle (generally to a value of around 180°), at a high angular velocity (of the order of 300° per second). The performance of the tire is evaluated as a function of the number of beads (0, 1 or 2) of the tire which have suffered unseating. Other parameters, such as the maximum level of transverse acceleration of the vehicle during the test or alternatively the deviation from the recommended trajectory may serve in evaluation of the unseating performance.
Although this method solves the problem of steering angle reproducibility, it nevertheless exhibits the drawback of being limited to a single steering angle, which reduces the quantity of usable data and the precision of ranking.
Consequently, there is a need for a tire testing method which makes it possible to determine the capacity of a tire to resist unseating and to draw up a ranking of a plurality of tires of different design.